1. Field of the Invention
This invention relates to a dry-type toner for use in electrophotography and electrostatic printing to develop latent electric or magnetic images, and more particularly to a dry-type toner which exhibits high fixing ability even at low image-fixing temperatures and can be used in a thermal image fixing method employing an oil-less heat-application roller.
2. Discussion of Background
Latent electrostatic images formed in the course of electrophotography, electrostatic printing and electrostatic recording are developed with, in the case of a dry method, a dry-type toner comprising as its main components a binder resin and a coloring agent. The developed images are transferred to the surface of a copy paper, and then fixed thereon.
Toner images can be fixed on a copy paper by various image fixing methods. Among them, a thermal image fixing method using a heat-application roller is widely adopted, because high thermal efficiency can be obtained and high-speed fixation can be achieved when this method is employed.
In order to achieve high-speed fixation by the thermal image fixing method, a toner used therein is required to exhibit high image fixing ability even at low fixing temperatures. Therefore, a resin having a low softening point is incorporated into the toner as a binder resin.
However, when such a resin is contained in the toner, the toner images partially stick to the surface of a heat-application roller during the process of image fixing, and the toner stuck to the roller-surface transfers to a copy paper to stain the background. This is a so-called off-set phenomenon. Furthermore, the copy paper tends to wind round the heat-application roller when the temperature of the roller is low. In this Specification, this phenomenon is referred to as a "winding phenomenon".
In order to eliminate the above adverse phenomena, the addition of a lubricant or releasant, such as a solid silicone, a varnish, a higher fatty acid, a higher alcohol or a wax of various kinds, has been proposed as disclosed in Japanese Laid-Open Patent Applications 51-143333, 57-148752, 58-97056 and 60-247250. However, toners containing any of the above lubricants do not exhibit high resistance to the off-set and winding phenomena while maintaining high fixing ability at low temperatures.
For instance, polyolefin waxes such as a low-molecular-weight polyethylene and a low-molecular-weight polypropylene, which have conventionally been used in a toner as a lubricant, can impart to the toner high resistance to the off-set phenomenon, but cannot sufficiently improve the fixing ability at low temperatures.
Vegetable waxes such as carnauba wax and candelilla wax can impart to a toner both high resistance to the off-set phenomenon and excellent fixing ability at low temperatures, but cannot impart to the toner high resistance to the winding phenomenon.
Solid silicones, varnishes, silicone oils, amide waxes, higher fatty acids, higher alcohols and montan wax can improve the fixing ability at low temperatures, but cannot sufficiently impart to a toner the resistance to the off-set and winding phenomena.
In addition, the conventional lubricants cannot be thoroughly dispersed in a binder resin, so that they tend to separate from the toner during the process of development, and stick to a photoconductor or a development sleeve. A so-called filming problem is thus caused. Furthermore, since a carrier tends to adhere to the toner containing the conventional lubricants, the toner cannot stably produce high quality images for a prolonged period of time.